*{
    margin:0;
    padding:0;
    box-sizing: border-box;
}

/*清楚浮动*/
.clearfix:after,
.clearfix:before{
    content:"";
    display:table;
    clear:both;
}
h1{
    text-align:center;
}
.content{
    width:1200px;
    margin:0 auto;
}
.main{
    background-color:#999;
    float:left;
    height:200px;
    width:25%;
    text-align: center;
}
@media (max-width:600px) {
    .content{
        width:100%;
    }
    .main{
        width:100%;
        background-color:yellow;
    }
}